Field of the Invention
The present invention relates to a fixing apparatus mounted in an image-forming apparatus.
Description of the Related Art
As a fixing apparatus mounted in an image-forming apparatus such as a copier and a laser beam printer, a fixing apparatus using a film is known. This fixing apparatus generally has a tubular film, a nip forming member that comes into contact with an inner surface of the tubular film, and a pressuring roller that forms a nip together with the nip forming member via a film. The fixing apparatus heats and fixes a toner image to a recording material while conveying the recording material through the nip.
Japanese Patent Application Laid-open No. H10-198200 discloses a fixing apparatus including a support member with a protruding portion extending in a longitudinal direction and protruding toward the pressuring roller to serve as a nip forming member. The protruding portion causes a portion of the nip to be locally highly pressured to enable an increase in a gloss value for a toner image fixed to glossy paper or the like. The protruding portion extends in the longitudinal direction, and the position of the protruding portion in a conveying direction of the recording material is at a given distance from the center of the nip in the conveying direction of the recording material.
On the other hand, an apparatus is known in which the width of the nip in the conveying direction of the recording material (hereinafter referred to as the width of the nip) varies in the longitudinal direction. By way of example, Japanese Patent Application Laid-open No. 2003-228246 discloses an apparatus in which the outer diameter of the pressuring roller increases gradually from a central portion toward ends of the pressuring roller in the longitudinal direction. When the fixing apparatus in which the width of the nip varies in the longitudinal direction is provided with the protruding portion described in Japanese Patent Application Laid-open No. H10-198200, the following problems result. The penetration level of the protruding portion into the pressuring roller is higher in an area corresponding to a relatively large nip width than in an area corresponding to a relatively small nip width. The difference in the penetration level of the protruding portion 1 corresponds to a difference in the pressure peak.
Thus, when the width of the nip is larger at ends of the pressuring roller than at a central portion thereof in the longitudinal direction, the pressure peak is higher at the ends than at the central portion. In contrast, when the width of the nip is larger at the central portion than at the ends, the pressure peak is higher at the central portion than at the ends.
When the pressure peak in the longitudinal direction varies, the gloss value for the toner image fixed on the recording material may vary. In an image, a large gloss value is achieved in an area where the toner image is fixed using a portion of the pressuring roller with a high pressure peak, whereas a small gloss value is achieved in an area where the toner image is fixed using a portion of the pressuring roller with a low pressure peak. This leads to uneven gloss. Such unevenness of the gloss value may be perceived as an image defect.